The present invention relates to an improved substrate for color proofing films which has a white, reflective, non-opaque, partially translucent side and an opposite non-opaque black or dark side. The composite substrate is used to produce color proofing films demonstrating reduced dot gain compared to opaque white substrates.
Color proofing systems are well known and are broadly disclosed in the prior art. The progressing from an original armwork through a series of color separations, and the use of these separations to produce color proofs is well known in the art. Color proofing films are used to assist a printer in correcting a set of color separations which will be used in exposing expensive lithographic printing plates. The proof should reproduce the image and color quality that will be obtained during the printing process. Visual examination of a color proof should reveal the color rendition to be expected from a press using the color separations, and any defects on the separations which might need to be altered before making the printing plates.
Color proofs can be made by using a printing press which is known as a proof press. This requires that all of the actual printing steps be formed and this conventional method of color proofing is costly and time consuming. Alternate color proofing methods have therefore been developed to simulate the quality of press proofs. There are two general types of photoimaging methods used to make a color proof, namely the overlay type and the single sheet type. In the overlay color proofing system, an independent transparent plastic support is used for producing a colored image of each color separation film. A number of such supports carrying the various colored images of cyan, yellow, magenta and black are then superimposed upon each other and placed on a white sheet to produce a color proof. An advantage of the overlay system is that the process is quick and economical. However, the transparent sheets between each color tend to distort the final image. In the single sheet color proofing method, a color proof is prepared by successively producing images of the colors from different color separation films onto a single receiver sheet. This is done by using a single opaque support and by applying toners, photosensitive solutions, or coatings of photosensitive materials of corresponding colors to the opaque support in succession. Examples of this approach are described in U.S. Pat. Nos. 3,671,236; 4,260,673; 4,366,223; 4,650,738; 4,656,114; and 4,659,642 which are incorporated herein by reference. An advantage of the single sheet type of color proof is that the color is not influenced by superimposed plastic supports. This method more closely resembles actual printing and eliminates the color distortion inherent in the overlay system. U.S. Pat. No. 4,596,757 provides a method for transferring images or solid colors which are subsequently imaged. The photosensitive material comprises a carrier support sequentially bearing a release layer; a colored, photopolymerizable layer; and an adhesive layer. The material can undergo exposure, lamination a temporary support, wet development, and then lamination to a final base sheet.
Images can also be produced by a peel apart method. Peel apart color proofing systems are known in the art as exemplified by U.S. Pat. No. 4,489,154. In peel apart color proofing, a photosensitive layer is disposed between two self supporting sheets. Upon exposure, there is an imagewise difference in adhesion between the exposed and unexposed portions of the photosensitive image to one of the two sheets forming the photosensitive element. Upon exposure and separation of the two sheets, a positive image is formed on one sheet and a negative image on the other.
Clearly, in the manufacture of color proofing films, it is desired that the images made from an original should represent a consistent reproduction of tonal values, and reproduce the quality seen in the originals. However, often, when these high quality color proofing separations are compared to the final output of a printing press, the results do not always correspond. This is because the printing press produces a halftone dot size on paper that varies from the halftone dots on the color proof. The result is what is called "dot gain". It manifests itself by the spreading out of ink on the printed sheet to cover an area different from that on the printing plate or color proofing originals. This problem has been recognized in the printing industry and techniques have been evolved to adjust for these variations. Dot gain can be useful in color proofing films because it simulates the mechanical gain which occurs on the press. However, there are situations where it is desired reduce the amount of dot gain. For example, when trying to simulate the dot gain of sheet fed presses or the printing done using positive working lithographic plates.
It has been found that the substrate on which the final color proofing image appears contributes to a difference in optical dot gain on the proof. This is disadvantageous since it contributes to the production of an image which is not a faithful reproduction of the original image. The prior art has recognized this problem and has sought to achieve a receiver base for image transfer which does not increase apparent dot gain. In one example, U.S. 4,579,810 discloses a multicolor proof having a pearlescent support. It has now been found that dot gain can be controlled using a partially translucent support which is white on one side and has a dark, preferably black surface on the opposite side. The white side allows an accurate reflective background base from which to view the colors of the image. White backgrounds, while themselves well known in the art are also transmissive to part of the light cast upon them. Since the background materials, usually white polyester films, have a certain thickness, they tend to reflect part of this transmitted light back through the image in a diffuse fashion and cause a shadow which is viewed as a gain in dot size. This invention solves this problem by providing the opposite side of the support with a dark, non-reflecting, preferably black surface. This causes the inevitable reflections to be much more coherent rather than diffuse, thereby substantially reducing shadow and hence dot gain.
One might assume that the same result could be attained by employing a white base which is completely opaque. However, it has been found that in order to correctly register sequential image separated halftone images as is necessary in color proofing, the receiver support must have a certain minimum amount of translucency. Importantly, the dark side also cannot be completely opaque, and must have a certain amount of capacity to transmit visible light. The white side cannot be too transmissive to light or the dark color from the underlying black surface will give the composite film a muddy appearance. The present invention provides an improved white receiver base for color proofing images which allows image registration and yet reduces optical dot gain to thereby allow a truer rendition of the original colored image. The base may either comprise a single polyester sheet with a white side and a black side, or it may be a laminate of a white film adhered to a black film. In one embodiment, the black side may have an additional white coating or white film juxtaposed with it so that two images may appear on opposite sides of a single document.